Unauthorized copying and redistribution of feature length movies represents a source of lost revenue for the movie studios. Advances in technology have made it easier to make and distribute unauthorized copies of such materials. There exists a need to identify unauthorized copies and to track them to the source of the copy. One method is to embed information in the image itself.
It is known that images or photographs can have information contained within the image area such as bar codes, date, and time. There have also been attempts to embed digital information, a digital watermark, in the image area, for example, for copyright protection. It is desirable to have such information embedded in pictorial images with minimal visibility under normal viewing conditions, while retaining the existing quality of the image. It is also desirable to have a method and an apparatus that can apply such a watermark at high speed to the color print film at the time of manufacture. The print film can then be tracked to a particular print of a given movie and to the theater in which that particular print copy was shown. The watermark itself would not necessarily prevent unauthorized copying, for example by a camcorder in the theater, but could provide a trace to the theater in which the copy was made.
U.S. Pat. No. 6,496,818 (Yoda), discusses embedding a pattern in a color print and adjusting cyan, magenta, yellow, black (CMYK) values such that the embedded data matches the color of the surround when viewed under a standard illuminant. Not only is such a technique directed at paper prints, a prior knowledge of the pictorial image data is required to accomplish the color matching. Such information would change from scene to scene, and would not be available to the film manufacturer at the time of manufacture.
U.S. Pat. No. 5,752,152 (Gasper et al.) presents another technique directed primarily at restricting unauthorized copying of photo studio paper prints. A pattern of microdots, less than 300 μm in diameter, is embedded into or deposited on the media. Typically, this watermarking is done by light exposure at the time of photographic paper manufacture. The restriction on maximum dot size helps to ensure that the array of dots is not visible to the unaided eye. Furthermore, the exposure is done with blue light such that the dots appear yellow when the paper is developed. Because a pictorial image is also present along with the array of microdots, the dot pattern becomes essentially invisible to the eye. Although an interesting concept, the pattern provides basically a “go, no-go” decision, allowing or preventing copying of the photograph. An identifier code, which can be varied for each copy of the movie, is needed for the movie industry.
U.S. Pat. No. 6,044,182 (Daly et al.) describes a method of embedding digital codes in an image that is combined with an encoding carrier image to produce a frequency dispersed data image. The frequency dispersed data image is added to a pictorial image to produce a source image containing embedded data. The frequency dispersed data image tends to mimic the grain noise in a photographic pictorial image. The embedded frequency dispersed image data can be laid down in tiles, e.g. 256 pixels by 256 pixels, with each tile containing the complete coded data. Thus, data can be spatially redundant within each frame to increase robustness.
Techniques similar to that described in U.S. Pat. No. 6,044,182 are being tested for use in digital cinema projectors. The frequency dispersed data image is applied to the digital image data at a low level in all three color channels, red, green, blue (RGB), in order to keep the embedded image neutral and not cause a hue shift. Spatial Light Modulator (SLM) resolutions up to QXGA (2048 pixels by 1536 lines) are available to allow a frequency dispersed data image to appear similar to film grain. Such a technique looks promising for digital cinema, but presents difficult exposure requirements for watermarking color print film.
It is desirable to pre-expose the color print film with the watermark as part of the film manufacturing process. At a particular point in the manufacturing process, the film transport speed is sufficiently well controlled where it is wrapped around a 6″ diameter drum. The film is moving at speeds up to 50 feet/second. To lay down a frequency dispersed image at high resolution at this speed without significant motion blur which would prevent subsequent detection of the embedded code, the exposure times may be as short as 100 ns. A 12 um pixel imaged on the film moving at 50 feet/second will experience an image motion smear of about 1.2 um or about 10% during a period of 100 ns. Minimizing smear or motion blur is important in being able to decode the embedded data reliably. A practical light source with sufficient radiance and power that can be turned on and off at such short times, coupled to an efficient writer optical system, to enable exposure at such writing speeds, represents a significant challenge.
An apparatus for exposing sensitometric (calibration) patches and barcode data onto color negative film during the manufacturing process is presented in U.S. Pat. No. 6,407,767 (Klees et al.) Two white light flashlamps, one large and one small are located in an integrating sphere and the resulting light is appropriately attenuated and directed through fibers to form what becomes gray scale patches on the processed film. These gray scale patches are used to control subsequent printing operations. The large flashlamp is repeatedly fired to approach the necessary exposure, and the exposure is then fine tuned by firing the small flashlamp, providing a very accurate exposure. A transmissive LCD is used in a separate optical system to provide the bar code. While this device can provide accurate color-neutral exposure onto stationary film as part of the manufacturing process, its power is not sufficient for exposing film at 50 feet/second, nor is its resolution sufficient to ensure the embedded data will not be easily seen by the eye by mimicking photographic grain.
U.S. Pat. No. 6,215,547 (Ramanujan et al.) discusses a writer with an Light Emitting Diode (LED) light source and a reflective LCD spatial light modulator with sufficient resolution to effectively expose a grain-like frequency dispersed data image. However, although the LED source may be capable of fast modulation speeds, it is not sufficiently powerful to deliver adequate exposure in any color at speeds significantly less than one second.
U.S. Pat. No. 6,330,018 (Ramanujan et al.) discloses that by using two LCD arrays, both of the orthogonal polarizations can be imaged. Such a construction, using identical image information on both LCDs, would effectively increase the efficiency of the optical system by a factor of two, while also increasing cost and complexity. A 2:1 increase in light at the film plane over that available from the device of U.S. Pat. No. 6,215,547 is still insufficient to expose color print film at times near 100 ns.
U.S. patent application Ser. No. 10/342,009 describes an improved LED light source over that of U.S. Pat. Nos. 6,215,547 and 6,330,018. However, it is estimated that even this light source structure, using four large area LEDs, modified and optimized for the watermarking application, would be inadequate in exposure by a factor of 30 to 40 for exposure times approaching 100 ns.
It is desirable to expose photographic media at very high media transport speeds at the time of manufacture with an essentially invisible watermark.